The present invention relates to a thermal recording system, which is particular, relates to an apparatus utilizing the periodic power source. This provides the constant printing concentration or density in spite of the change of the voltage of the power source, in an inexpensive high-speed thermal recording system.
A thermal recording system utilizes a thermal head having a series of aligned heat-generating resistor elements. An electric current is applied to each to the heat-generating resistor elements in accordance with the black or white information of a picture cell to be recorded, so that the Joule heat thus generated in the resistor elements is transferred to a heat-sensitive treated thermal paper in close contact with the thermal head for color formation. Such a heat sensitive recording system has been used in fascimile receiver and/or a computer terminal device and the like. An example of the thermal head and the apparatus for supplying the electric power to said thermal head has been shown in the U.S. Pat. No. 3,609,294.
The heat-sensitive color formation theoretically requires a comparatively long time (about 1 to 10 mS) for recording, so that, when high-speed recording is required a simultaneous multi-dot recording system which simultaneously records a plurality of dots is used. For the simultaneous recording, it is necessary to hold the current through a plurality of elements for a period required by the color formation, and current-holding elements, such as flip-flop circuits, SCR, and/or thyristor circuits, are used for that purpose. Especially, when a very-high-speed recording is desired, the number of elements to be recorded simultaneously is increased, and the number of the corresponding current-holding elements is also required to increase. For example, in the case of a thermal head having heat-generating resistor elements which are aligned along a straight line over the entire recording width of a recording medium, let it be assumed that the recording width is 256 mm, the recording density is 8 dots per mm, the recording time normally required is 10 mS, and the recording time of one line is 40 mS. Then, the total number of the heat-generating resistor elements becomes 8.times.256=2,048, and the number of recordng operations per one line becomes 40.div.10=4. So that in each current application for each recording operation, the electric current through up to 2,048.div.4=512 heat-generating resistor elements must be held depending on the picture to be recorded. More particularly, the thermal head with 2,048 pieces of heat-generating resistor elements is divided into four blocks, 512 current-holding elements are successively connected to each block, and the recording system is so constructed as to give 512-bit picture signals. However, the number of the wirings for the signal lines become too large to be practical. Accordingly, an integrated circuit structure has been devised, in which the thermal head, the current-holding elements, and a matrix circuit for arranging input signals thereto in a matrix form to reduce the number of signal lines are integrated.
FIG. 1 illustrates a simplified schematic diagram of an example of the previously mentioned thermal head, which uses thyristors (SCR) as the current-holding elements and a matrix circuit comprising AND circuits as input circuits for the gates of the thyristors. In FIG. 1, a plurality of heat-generating resistor elements 1 are aligned along a straight line, and one side terminals of all the heat-generating resistor elements are connected in common to a power source terminal 2. The opposite side terminals of the heat-generating resistor elements 1 are connected to anodes (A) of thyristors 3 respectively, and the cathodes (K) of all the thyristors 3 are connected to an earth terminal 4 in common. The gates (G) of the thyristors 3 are connected to the joints of resistors 5 and anodes of two diodes 6 and 7, while the opposite ends of all the resistors 5 are connected to a bias voltage source terminal 8 in common. The cathodes of the diodes 7 are grouped by consecutive n pieces in common and connected to selector terminals 9, as shown in the figure. The cathodes of the other diodes 6 are grouped by taking every n'th pieces in common and connected to driver terminals 10, as shown in the figure. Accordingly, if there are a pieces of the heat-generating resistance elements in total, then the number m of the selector terminals 9 will be m=a/n, and the input signals in this example are arranged in the form of an m.times.n matrix.
It is noted here that, as apparent from the foregoing illustration, if the total number of the heat-generating resistance elements 1 in the thermal head is a, then the total number of the thyristors 3, the total number of the resistors 5, the total number of the diodes 6, and the total number of the diodes 7 will be also a, respectively. As regards the number of the input lines, there is one power source terminal 2, one earth terminal 4, one bias voltage source terminal 8, the selector terminals 9 (m lines), and the driver terminals 10 (n lines), so that (m=n=3) lines in total.
The illustrated example of the thermal head is of hybrid-type IC (integrated circuit) construction, which comprises the heat-generating resistor elements 1 formed on a ceramic substrate by a thin or a thick film and aligned along a straight line; m pieces of silicon chip IC are mounted on the aforesaid ceramic substrate, each of which silicon chip IC includes n circuit groups integrated thereon, each of the said circuit groups consisting of one of the thyristors 3, one of the resistors 5, one of the diodes 6, and one of the diodes 7; and three-dimensional wirings which provide connections relating to the diodes 6 and 7.
It should be noted in FIG. 1 that a pair of diodes 6 and 7 operate as an AND circuit, which provides the output signal only when one of the selector terminals 9 and one of the driver terminals 10 are simultaneously supplied the input signals. When the AND circuit composed of the pair of diodes 6 and 7 provides the output signal, the relating thyristor 3 connected to the opened AND circuit is conducted and then the relating resistor element 1 is heated by the electric current which flows from the power source terminal 2 through the resistor elements 1, and the thyristor 3 to the earth terminal 4.
Preferably, the value (m) is 64, and the value (n) is 32, thus, the number of the resistor elements in a whole horizontal dot line is (m).times.(n)=64.times.32=2,048.
FIG. 2 shows the outline of an example of circuits for driving the thermal head of FIG. 1. The operation of the thermal head will be described by referring to FIG. 2.
Picture signals 11 are time sequentially applied to a shift registor 14 which has n number of bit position, in the order of the alignment of the heat-generating registor elements 1 (FIG. 1) in the thermal head 12 (The thermal head 12 is the same as that shown in FIG. 1). Clock signals 13 corresponding to each picture element (dot) of the picture signals 11 are applied to the clock terminal of the shift registor 14, while the aforesaid picture signals 11 are applied to the serial input terminal of the shift registor 14, so as to be successively stored therein in accordance with the clock signals 13. The shift registor has n steps, and the output from those steps are successively connected to the driver terminals 10 of the thermal head 12 so as to match the alignment of the corresponding heat-generating resistor elements 1 (for n lines). The clock signals 13 are also applied to a counter 15, and upon counting n clock signals, the counter 15 generates a carrier signal 16 which is similar to one clock signal, and the carrier signal 16 is applied to another counter 17. The outputs from different steps of the counter 17 are connected to a decoder 18, and the outputs from the decoder 18 are successively connected to the selector terminals 9 of the thermal head 12 so as to match the alignment of the corresponding heat-generating resistor elements 1 (for m lines).
The power source terminal 2 of the thermal head 12 is connected to one end of the secondary winding of a transformer 19 for transforming the commercial AC power source voltage to a level required by the thermal head. A detector circuit 20 is also connected to the power source terminal 2, for checking whether the voltage at the power source terminal 2 is above the holding voltage for ensuring the holding current of the then conductive thyristor 3. The output 21 from the detector circuit 20 is connected to a control circuit (not shown), which triggers the picture signal 11 and the clock signal 13 upon detection of the output of the detector circuit 20.
The bias voltage source terminal 8 of the thermal head 12 is connected to the positive polarity terminal of a bias DC voltage source 22. The earth terminal 4 of the thermal head 12, the other end of the secondary winding of the transformer 19, and the negative polarity (earth) terminal of the bias voltage source 22 are commonly connected to the earth line (the grounded symbol of the figure) of the illustrated circuit.
In the operation of the circuit of FIG. 2, the detector 20 informs, upon detection, the control circuit that the voltage of the power source terminal 22 has increased above the holding voltage for ensuring the holding current of the thyristor 3. Accordingly, the control circuit causes the picture signals 11, which alternatively assumes one of black/white two levels, i.e., "0" level for white and "1" level for black, to be applied to the circuit of FIG. 2 simultaneously with the clock signals 13, so that the data representing n picture elements of the picture signals are stored in the shift register 14. At this moment, the counter 15 produces a carrier signal 16, indicating that picture elements are stored in the shift register 14 to its full capacity. The counter 17 is actuated for causing the decoder 18 to produce a pulse signal at one of the output lines thereof.
In the case of the thermal head of FIG. 1, if it is assumed that the picture signals are applied to the heat-generating resistor elements 1 in succession starting from the extreme left element of the figure. The first pulse signal from the decoder 18 is applied to that output line thereof which corresponds to the selector terminal 9 relating to the extreme left group heat-generating resistor elements of FIG. 1. Regarding the output signals from the shift registor 14, the right side steps of the shift registor 14 of FIG. 2 are connected to those driver terminals 10 which relate to the left side heat-generating registor elements of FIG. 1, so that those picture signals 11 which are applied to the shift registor 14 time-wise earlier correspond to the left side heat-generating registor elements of FIG. 1.
When the pulse signals are applied to the selector terminal 9 and the driver terminal 10, the AND condition of the AND circuit composed of the diodes 6 and 7 are satisfied. When this occurs, the output signal of said AND circuit is applied to the gate terminal (G) of the thyristor 3, then said thyristor 3 is turned on.
On the other hand, those thyristors with which thyristors the contents of the shift register 14 do not meet the aforesaid AND conditions, are retained in an OFF state. When the data of the picture signals 11 corresponding to the next n picture elements are stored in the shift registor 14, the decoder 18 similarly generates another pulse signal at the output. This pulse signal is now applied to the selector terminal 9 relating to the second group thyristors from the extreme left as seen in FIG. 1 (as a result of the counting by the counter 17).
Accordingly, the picture signals of n picture elements at this moment relate to the n picture elements of the second group thyristors from the extreme left, so that the thyristors 3 belonging to said second group are either turned ON or kept Off. Other thyristors 3 are successively triggered group by group in a similar manner. Thus, for instance, 512 picture signals 11 are applied to and held by the thyristors 3. Then, the voltage at the power source terminal 2 changes along the positive half cycle of sinusoidal waveform and approaches to 0 V and, the corresponding output signal 21 from the detector circuit 20 is applied to the control circuit. As a result, the then conductive thyristors 3 (corresponding to colored dots) are all turned OFF due to the current reduction below the holding current, so that the picture signals stored in the thyristor 3 are erased. Since the mentioned operations of the shift registor 14, the counter 15 and the like can be effected in the high speed electronic circuit in about less than 1 .mu.S, if it is assumed that the number of the heat-generating resistance elements 1 in one of said groups (the aforesaid n) is assumed to be 32, the operation for one group is finished in 1 .mu.S.times.32=32 .mu.S. If the number of picture signals to be simultaneously recorded is 512 dots, such picture signals 11 can be set on the thyristors 3 for holding within 1 .mu.S.times.512=512 .mu.S. If the commercial power frequency is 50 Hz, the duration of one half cycle is 10 mS, which is long enough for ensuring the thermal head to generate heat for recording. Since the relation of 512 .mu.S&lt;10 mS is satisfied, the density of color formed for recording is constant and even regardless of the time difference in turning on the different thyristors. This is to say, the time 512 .mu.S is negligibly short compared with 10 mS, and the operation time for turning on the thyristors 3 does not substantially affect the density of printed color.
In FIG. 3, the waveform (a) is that of the voltage at the power source terminal 2, and the wave form (b), is that of the output 21 from the detector circuit 20. The period of one cycle of the commercially power source voltage is represented by T.sub.o, which is 20 mS for the commercial power frequency 50 Hz. The level of the holding voltage at the power source terminal 2 is represented by V.sub.t, which level V.sub.t is necessary for passing the holding current through the thyristor 3. Accordingly, the time period t.sub.v is for heat generation by the thermal head 12, and the time period t.sub.s is for erasing the picture signals stored in the thyristors 3 (5 to 10 .mu.S) and for pausing. It is noted here that, in the illustrated wave-forms, the time for setting the picture signals on the thyristors 3 (512 .mu.S in the above example) is included in the time period t.sub.v.
However, the thermal printer system shown in FIG. 2, has the disadvantage in that the printed concentration or density depends upon the fluctuaton of the voltage of the commercial power source. The voltage of the commercial power source is generally not stable enough as to maintain the constant density of the printed color. If we wish to obtain the constant density of the printed color utilizing the commercial power supply, we must use an expensive automatic voltage regulator in an alternate current stage, or we must use a voltage regulated direct current (DC) power supply. However, both have the disadvantages that their prices are expensive and the size of devices is large.